Polyamides are well known and useful materials utilized throughout the world. The variations in their material properties are influenced by their polymer structures. Their polymer structures are influenced by the chemical nature and structure of the polymer's monomer units and the way in which the monomer units are joined in the polymer.
The term nylon is embraced within the broader class of polyamides. It is most commonly applied to those polyamides which are melt processable such as nylon 6,6 and nylon 6. Such thermoplastic polyamides are distinguishable from polyamides which are not melt processable such as polyparaphenylene terephthalamide. The monomers for polyamides are generally either a diamine and a dicarboxylic acid or an amino acid. A diamine and a dicarboxylic acid lead to a nylon noted as AABB such as nylon 6,6 made from 1,6 diaminohexane and adipic acid. An amino acid or its lactam leads to a nylon noted as AB such as nylon 6 made from 6-aminohexanoic acid or its lactam, caprolactam.
The largest usage AB type nylon is that made from caprolactam which is known worldwide as nylon 6. This polymer has a relatively simple chemical structure. Its structure is an aliphatic repeat unit of 5 methylene units joined by an amide unit. Homologs of this polymer have also been explored and developed. Two homologs which are commercially available polymers are nylon 11 and nylon 12 in which the number of methylene groups is 10 and 11 respectively. These nylons have a longer methylene chain length and a lower ratio of amide groups to methylene groups than nylon 6,6 or nylon 6. Polymers such as nylon 11 and nylon 12 derive much of their value from their reduced moisture sensitivity compared to nylon 6,6 or nylon 6. However, the longer methylene component of these polymers also results in reduced melting points such as about 190 C and 180 C for nylon 11 and nylon 12 respectively.
These melting points can be compared to about 225 C for nylon 6 and 265 C for nylon 6,6. Their lower melting points limit these longer chain nylons from applications in which they might otherwise provide a good balance of properties. Such applications are many industrial fiber applications or molded articles in high temperature environments. Melt processable polymers which have the properties to fit these more stringent thermal and mechanical requirements are often termed engineering thermoplastics.
Polymers in the categories of nylons, polyolefins, and polyesters derive many of their useful properties from being semi-crystalline materials. This term denotes the fact that in the solid state many of the chains of these polymers align into spatially regular structures which are crystalline regions. Such regularity is revealed by, for example, patterned diffraction of x-rays. The remainder of the polymer chains are disordered forming what is termed the amorphous regions. The amount of crystalline content of these polymers covers a wide range with most falling within the range of approximately 20% to 80%.
Useful semi-crystalline engineering thermoplastics are bounded by both lower and upper melting points. Lower melting points limit applications whereas higher melting points introduce problems such as increasing degradation at the higher temperatures required in processing and fabrication. A useful range for the melting temperature for a wide variety of engineering thermoplastics is approximately 200 C to 300 C.
Partial substitution of an alternate minor monomer for the predominate monomer in semi-crystalline polymers normally results in a reduction in melting point. One somewhat uncommon phenomenon occurs in which a second monomer unit fits into a polymer's crystal lattice in the place of a predominant first monomer. Substitutable monomers of this type are often termed isomorphous (Kohan, pp. 370–374). The substitution of an isomorphous monomer normally produces a monotonic and continuous melting point behavior over the range of monomer concentrations.
One approach to increasing the melting points of aliphatic nylons is the incorporation of comonomers with rigid units such as phenyl or cyclohexyl into the polymer chain. If these units are isomorphic with the aliphatic units which they replace, the melting point of the polymer can be increased with little, if any, sacrifice in crystallinity (J. Ridgway, J. Polym. Sci., A-1, vol. 8, pp. 3089–3111, 1970). This phenomenon has been explored much more widely in the AABB type nylons than in the AB type nylons.
A desirable goal would be a family of AB type nylons with longer methylene lengths having reduced moisture sensitivity such as those of nylons 11 and 12 but with higher melting temperatures. Such higher melting temperatures would allow use in higher temperature applications. This invention describes compositions which achieve an improved combination of lower moisture absorption with a useful range of melting points in a family of AB type nylon polymers.